In modern projection objectives for microlithography, use is made of a large number of wavefront manipulators for the correction of optical aberrations. Most of these manipulators bring about a wavefront correction by mechanical manipulation of optical elements. This is done by a position change and/or by a deformation of the optical elements. These manipulators have correction possibilities for low-order wavefront aberrations such as typically occur when the objectives are used in conjunction with so-called conventional settings and at a throughput of approximately 120 wafers/hour.
However, constantly increasing desired throughput properties involve ever higher light intensities in the objective and hence a constantly rising thermal load that acts on the optical elements. This thermal load causes wavefront aberrations: in the case of lenses by way of the temperature-dependent refractive index and the surface deformation; in the case of mirrors predominantly as a result of the surface deformation on account of the thermal expansion of the mirror substrate. In addition, in the development of lithography, the trend is towards illumination settings, such as the dipole settings, for example, which entail a strong focusing of the light power density on, in particular, lenses near the pupil and therefore, on account of the resultant locally high thermal load, can also cause radial and/or azimuthal higher-order and highly localized wavefront aberrations.
One possibility of compensating for the wavefront aberrations caused by lifetime effects in a more efficient manner is currently the use of exchangeable plates to which special correction aspheres are applied. Within the lifetime of the objective, these compensation plates can be exchanged repeatedly in order to cope appropriately with the changes in the wavefront aberrations during the lifetime of the objective.
Although compensation plates with correction aspheres can compensate for aberrations, they are rather unsuitable for the compensation of dynamically rapidly variable aberrations. Moreover, the aberration to be compensated for has to be known before the creation of the compensation plate, and in particular therefore before the compensation plate is incorporated into the projection objective. Since new wavefront aberrations are in turn induced by the incorporation of the compensation plate, naturally complete compensation is not possible here.
As already mentioned, mechanical manipulators are known. Thus, DE 198 24 030 A1, for example, describes a catadioptric projection objective with adaptive mirrors, wherein mirrors can be deformed by actuating elements in such a way that specific image aberrations are reduced.
EP 678 768 and DE 198 59 634 A1 likewise disclose projection exposure apparatuses in which lenses or mirrors are deformed by actuators for image aberration correction.
Since mechanical elements in the optical beam path cause shading and scattered light, however, mechanical concepts, in the case of lenses to be manipulated, are restricted to the manipulation of the lens edge. This restriction to the lens edge can constitute an inherent limitation of the possible correction profiles, and especially of the radial orders, which is unable to be circumvented even by complex mechanisms.
As an alternative to the mechanical manipulators, thermal manipulators are known, wherein the thermal manipulators are likewise arranged at the lens edge, such as in the U.S. Pat. No. 6,198,579 B1, for example. However, the thermal manipulators proposed in the cited document can exhibit the same limitations in the radial orders as their mechanical counterparts and additionally imply relatively long time constants given by the propagation speed of the heat over the lens diameter. Edge-actuated thermal manipulators are therefore predominantly suitable for compensation of temporally steady-state wavefront aberrations. On account of the long time constants, however, such manipulators may be suitable only to a very limited extent for the compensation of transient wavefront aberrations.
Furthermore, a method for the correction of non-rotationally symmetrical image aberrations with Peltier elements arranged at the periphery of lenses is known from DE 198 27 602 A1, wherein the Peltier elements influence the thermal behavior of the optical element in such a way that in the case of non-rotationally symmetrical passage of radiation through the element, resultant imaging aberrations can be corrected.
A device and a method for the correction of asymmetrical thermal loads of an optical element such as a lens or a mirror are likewise known from DE 198 59 634 A1, wherein the optical element is likewise deformed by actuators. It is likewise known from U.S. Pat. No. 6,081,388 to deform surfaces of lenses by actuators or defined mechanical forces in such a way that the imaging aberrations are influenced.
Furthermore, it is known from U.S. Pat. No. 6,521,877 B1 to influence the temperature of an optical element locally by transparent resistive layers. An alternative approach is disclosed in U.S. Pat. No. 6,466,382 B2, which proposes applying on a lens layers having absorbent properties which have a structure complementary to the footprint of the useful light.
The documents US2007/0019305 A1, US2003/0021040 A1, WO2006/128613 A1, JP2004/246343 A1, EP0678768 A2, U.S. Pat. No. 6,198,579 B1 and DE 10 2005 062401 A1 disclose further concepts for improving the imaging properties of optical systems such as e.g. projection objectives for semiconductor lithography.
WO 2004/036316 discloses a method for the correction of imaging aberrations of optical elements such as mirrors and lenses wherein, by additional irradiation, the temperature of the optical elements is altered in such a way that the imaging aberrations are reduced. However, the temperature of the optical elements can increase overall as a result of the additional irradiation, which can have a negative effect on the possibilities for using the concept disclosed in the cited document; in particular the disturbance of adjacent lenses and structures (mounts, manipulators, . . . ). The effect can be significantly increased imaging/wavefront aberrations.
A further document that realizes the refractive index and/or the shape of a lens by thermal influencing of the lens is US2006/0244940 A1, wherein infrared light is radiated laterally into the lens to be thermally influenced. However, this can involve a constrained arrangement of the optical waveguides outside the optically utilized area of the lens, and thus also the a priori far distance of the manipulator from the lens to be manipulated. In particular the indefinite heat dissipation of the lens manipulated in this way is disadvantageous.